Purification of chloroplast cytochrome b 559

A mutant strain of Scenedesmus obliquus deficient in ribulose diphosphate carboxylase, cytochrome f and photosystem II activity

The partial reactions of photosynthesis shown by strain F208, a non-photosynthetic mutant strain of Scenedesmus obliquus, have been compared with those performed by other mutant strains which lacked; Photosystem II activity (strains 11 and F131), cytochrome f (strain 50), P-700 and cytochrome f (strain F 119), and P-700 (strains F139 and 199). In this respect the properties of strain F208 were those that would be expected if Photosystem II activity and cytochrome f were not present in this strain. Examination of the composition of strain F208 has shown the absence of cytochrome f in both the soluble and the membrane-bound form. The considerably lower level of plastoquinone compared to that found in the wild type is characteristic of the strains which lack Photosystem II activities. Fraction 1 protein could not be detected in extracts of strain F208 by sedimentation velocity experiments in the ultracentrifuge, and only 7% of the wild type ribulose diphosphate carboxylase activity was found after chromatography of these extracts on DEAE-cellulose. The properties of strain F208 are compared with those of the ac-20 and cr-1 strains of Chlamydomanas rheinhardi, both of which have a deficiency of ribulose diphosphate carboxylase which is considered to result from a deficiency of chloroplast ribosomes. Strain F208 resembles these strains in its abnormal chloroplast ultrastructure and its decreased levels of the RNA forms derived from the chloroplast ribosomes when compared with the wild type. Chloroplast fragments isolated from strains of S. obliquus which lacked cytochrome f (strains 50 and F208) were able to use diaminodurene and ascorbate as an electron donor to Photosynstem I. Since this reaction was inhibited by mercuric salts it would appear that plastocyanin, but not cytochrome f, was involved in this electron transfer.

Chloroplast cytochrome b-6. Molecular composition as a lipoprotein

Disc electrophoretically homogeneous spinach-chloroplast cytochrome b-6 was found to be a lipoprotein whose redox potential was essentially unchanged during isolation. These results further support the hypothesis of Triton X-100/4 M urea, pH 8, as a useful extracting medium for membrane lipoproteins. Cytochrome b-6 was found to have a heme equivalent dry weight of 1 mol of heme per 60000 g. Of this, 20000 g was lipid-extractable. The molecular weight was 60000 with a partial specific volume of 0.84 ml/g. The protein portion of the molecule (40000) consisted of 1 polypeptide chain of 20000 daltons, 1 of 9600 daltons and 2 of 6600 daltons. A simple lipid composition (relative to the original membrane) was found consisting of 7 mol of chlorophyll a and 6 mol of cardiolipin per mol of cytochrome; these two lipids thus account for about 75-80% of the lipid content. An unidentified minor neutral lipid and minor polar lipid were also detected. At pH 7.0 in the presence of 0.5% Triton X-100, E'-o was -0.080 V, and in the absence of Triton X-100, E'-o was -0.120 V. At pH 8 in 0.5% Triton X-100, E'-o was -0.084 V, thus indicating that the redox potential is independent of pH in the region 7-8. The redox reaction proceeded via a one-electron-transfer.